The present invention relates to a chuck for releasably gripping a tool bit and, more specifically, to a keyless, self-tightening chuck which increases the gripping force on a tool bit in response to the torque applied through the tool bit to the work piece.
Various types of chucks are known for releasably gripping the stem of a tool bit such as a twist drill. The most common chuck in use is the so-called Jacobs chuck which includes a nosepiece and an adjusting sleeve that is adjusted to cause gripping jaws located within the nosepiece to forcibly grip the drill bit. The adjusting sleeve typically includes a plurality of axially extending gear teeth that are adapted to cooperate with a gear-type key to permit manual tightening and loosening of the jaws. The Jacobs-type chuck generally provides satisfactory performance although the requirement for a separate adjusting key necessitates a minimum level of manual skill to effect proper tightening, since, if the jaws are not tight enough, the drill bit will rotate relative to the jaws during the drilling operation and cause spalling of the drill bit stem. Also, the use of a key to tighten and loosen the chuck increases the time needed to insert or remove a tool bit, and, from a more practical standpoint, the key represents another part that is subject to mechanical wear with repeated use and another part which the chuck user must keep track of.
A number of drill chucks, known generically as keyless chucks, have been designed in an effort to eliminate the need for the tightening key. These chucks are adapted to apply an initial, hand-tightened gripping force to the drill bit with the gripping force self-tightening or proportionally increasing in response to the torque applied through the drill bit during the drilling operation.
Some of these self-tightening chucks have been provided with gripping jaws which toggle in response to the reaction torque encountered by the drill bit, and still others have been provided with cam-like surfaces which contact and cooperate with the gripping jaws to increase the gripping force. While most self-tightening chucks operate in a generally satisfactory manner, in some designs the portion of the gripping force that results from the self-tightening feature varies with the diameter of the particular drill bit being gripped in the chuck. In addition, the ratio of self-tightening gripping force to the hand-tightened gripping force can be undesirably low, that is, the portion of the total gripping force that results from the auto or self-tightening feature of the chuck is smaller than the portion that results from the initial hand-tightening of the chuck. Since most keyless chucks are designed for industrial rather than consumer applications, they tend to be fabricated as relatively large, machined assemblies that are expensive to produce.
One example of a self-tightening keyless chuck is shown in U.S. Pat. No. 2,910,302 to E. Ondeck. The chuck disclosed therein includes a tool receiving nosepiece that cooperate with wedge-like jaws to grip a tool bit. The nosepiece is rotated by hand to apply an initial hand-tightened gripping force to the tool bit with the self-tightening gripping force provided by a cam plate that includes cam tracks that are operative for one direction of rotation of chuck and inoperative for the other direction of rotation.
While this chuck is generally satisfactory in operation, the axial thrust force and the consequent self-tightening of the jaws is produced in response to a uni-directional reaction torque, that is, the desirable self-tightening feature occurs only for rotation in one direction with rotation in the opposite direction causing the chuck to loosen and release the drill bit. Also, from a practical standpoint, the chuck is fabricated from rather large, machined elements which increase the manufacturing costs of the chuck.